1. Field of the Invention
The present invention relates to a charged-particle beam system equipped with a device for correcting chromatic and spherical aberrations, such as an electron beam system (e.g., a scanning electron microscope) or an ion beam system (e.g., an ion microprobe).
2. Description of Related Art
In a scanning electron microscope or transmission electron microscope, an aberration corrector is incorporated in the electron optics in order to provide high-resolution imaging or to enhance the probe current density. One proposed example of this aberration corrector uses a combination of electrostatic quadrupole elements and magnetic quadrupole elements to correct chromatic aberration. The corrector also uses four stages of octupole elements to correct spherical aberration. The principle is introduced in detail in the following references: V. H. Rose, Optik 33, Heft 1, 1-24 (1971); J. Zach, Optik83, No. 1, 30-40 (1989); J. Zach and M. Haider, Nucl. Instr. and Meth. In Phys. Res. A 363, 316-325 (1995); and M. Haider et al., Optik 63, No. 1, 9-23 (1982)
The principle of the above-described aberration corrector is described briefly now by referring to FIG. 1, where an aberration corrector C is placed ahead of an objective lens 7. The aberration corrector C comprises four stages of electrostatic multipole elements 1, 2, 3, 4, two stages of magnetic quadrupole elements 5, 6, and four stages of electrostatic octupole elements 11, 12, 13, 14. The two stages of magnetic quadrupole elements 5, 6 create a magnetic potential distribution analogous to the electric potential distribution created by the second and third stages of the electrostatic multipole elements to produce a magnetic field superimposed on the electric field. The four stages of electrostatic octupole elements 11, 12, 13, 14 create an electric field superimposed on the electric field created by the four stages of electrostatic quadrupole elements.
Superimposition of electric potentials, superimposition of magnetic potentials (magnetic excitations), and superimposition of electric and magnetic potentials are next described by referring to FIGS. 2 and 3.
In the case of an electrostatic design, a final stage of power supplies An(n=1, 2, . . . , 12) capable of supplying a voltage to 12 electrodes Un(n=1, 2, . . . , 12) independently is connected as shown in FIG. 2. Where a quadrupole electric field is produced, output voltages from a quadrupole power supply 10 are supplied to the final-stage power supplies An to obtain a field close to an ideal quadrupole electric field. If it is assumed that the output voltages from the final-stage power supplies An are proportional to the output voltages from the quadrupole power supply 10, the ratio of the output voltages from the power supply 10 assumes a value as given in the reference to M. Haider et al. referenced above. Where an octupole electric field is created such that it is superimposed on the quadrupole electric field, output voltages from an octupole power supply 18 are added to the output voltages from the quadrupole power supply 10 and supplied to the final-stage power supplies An to obtain an electric field close to an ideal octupole electric field. Similarly, an electric field on which a multipole electric field produced by a 2n-pole element (n=1, 2, . . . , 6) is superimposed is obtained using the single dodecapole (12-pole) element.
In the case of a magnetic design, a final stage of power supplies Bn (n=1, 2, . . . , 12) capable of supplying excitation currents to the coils on 12 magnets Wn (n=1, 2, . . . , 12) independently is connected as shown in FIG. 3. Where a quadrupole magnetic field is created, output voltages from a magnetic quadrupole-element power supply 15 are supplied to the final stage of power supplies Bn to produce a magnetic field close to an ideal quadrupole magnetic field. If it is assumed that the output currents from the final-stage power supplies Bn are proportional to the output voltage from the magnetic quadrupole-element power supply 15, the ratio of the output voltages from the power supply 15 assumes an exciting magnetic force ratio as given in the reference to M. Haider et al. referenced above. In the description provided so far, superimposition of multipole magnetic fields other than a quadrupole magnetic field has not been explained. However, a multipole magnetic field can be superimposed in the same way as in the electrostatic design, by adding voltages for multipole magnetic fields to the input voltage to the final-stage power supplies Bn. A yoke for magnetically connecting the outside portions of the magnets Wn is omitted in FIG. 3.
Where electrostatic and magnetic designs are superimposed, a conductive magnetic material may be used so that the magnets Wn can act also as the electrodes Un. In this case, the coils on the magnets are mounted so as to be electrically isolated from the electrodes.
In the description given below, the 2n-pole elements are treated as if they were stacked on top of each other to simplify the explanation. In practice, superimposition of multipole fields on a single dodecapole (12-pole) field is achieved by adding voltage signals as mentioned previously.
A multipole element that is equivalent in function to a structure having reference electrodes in the X-direction is usually known as a normal 2n-pole element (n=1, 2, . . . , 6). A multipole element that is equivalent in function to a structure obtained by rotating the normal 2n-pole element through an angle equal to 1/2(=2π/4n=π/2n[md] or 90/n [deg]) of the pitch angle of the electrodes is known as a skew 2n-pole element. Similarly, in the case of magnetic type, a multipole element that is equivalent in function to electrodes of an electrostatic skew 2n-pole element is known as a normal 2n-pole element. A multipole element that is equivalent in function to a structure obtained by replacing the electrodes of the electrostatic normal 2n-pole element by magnetic pole pieces is known as a skew 2n-pole element. The electrostatic design and magnetic design are different in arrangement of electrodes and magnetic pole pieces in normal multipole element or skew multipole element, because the directions in which charged particles undergo forces from these fields are selected to be aligned on the same straight line. In the following description, these electrodes and magnetic pole pieces (or magnetic poles) may be referred to as pole elements in a case where it is not necessary to discriminate between the electrodes and magnetic pole pieces.
To conveniently discriminate between different ways of mounting multipole elements to particle beam equipment, in a case where the straight line connecting pole elements to which electric potentials U1 and U7 are applied as shown in FIG. 2 is coincident with the X-direction, the multipole elements are referred to as multipole elements (12-pole elements) of normal arrangement. Where a straight line connecting midpoints between these pole elements and adjacent pole elements is coincident with the X-direction, the multipole elements are referred to as multipole elements (12-pole elements) of skew arrangement. A multipole element of skew arrangement can be used as a multipole element of normal arrangement if the method of applying potentials to the pole element is varied (see M. Haider et al. referenced above).
Actual operation performed using the above-described four multipole elements 14 is next described by referring to FIG. 1. A normal dipole element acts as a deflecting device in the X-direction. A skew dipole element acts as a deflecting device in the Y-direction. These are used for axial alignment. Description of their details is omitted herein.
Adjustment of the focus of a beam of charged particles, i.e., formation of a reference trajectory, is first described. In the configuration of FIG. 1, the beam of charged particles enters from the left side as viewed in the plane of the sheet. A reference trajectory for the beam of the charged particles is created by four stages of electrostatic quadrupole elements 1, 2, 3, 4 and by an objective lens 7. The beam is focused onto a surface 20 of a specimen. In FIG. 1, both trajectory Rx in the X-direction of the particle beam and trajectory Ry in the Y-direction are drawn schematically on the same plane.
The reference trajectory is taken as a paraxial trajectory (i.e., a trajectory assumed when there is no aberration). The quadrupole element 1 causes the Y-direction trajectory Ry to pass through the center of the quadrupole element 2. The quadrupole element 2 causes the X-direction trajectory Rx to pass through the center of the quadrupole element 3. Finally, the quadrupole elements 3, 4 and objective lens 7 cause the beam of charged particles to be focused onto the specimen surface. In practice, these need to be adjusted mutually for complete focusing.
Referring more particularly to FIG. 1, the charged-particle beam in the X-direction trajectory Rx is diffused by the quadrupole element 1 acting like a concave lens. Then, the beam is converged to the center of the quadrupole element 3 by the quadrupole element 2 acting like a convex lens. The beam is thus made to pass through the center of the quadrupole element 3. Then, the beam is focused by the quadrupole element 4 and travels toward the objective lens 7. On the other hand, the charged-particle beam in the Y-direction trajectory Ry is focused to the center of the quadrupole element 2 by the quadrupole element 1 and made to pass through the center of the quadrupole element 2. Then, the beam is focused by the quadrupole element 3. Finally, the beam is diffused by the quadrupole element 4 and moves toward the objective lens 7. In this way, the function of a single concave lens is created by combining the diffusive action of the quadrupole element 1 acting on the X-direction trajectory Rx and the diffusive action of the quadrupole element 4 acting on the Y-direction trajectory Ry.
Correction of chromatic aberration using the aberration corrector C is next described. To correct chromatic aberration by the system shown in FIG. 1, the potential φq2 [V] at the electrostatic quadrupole element 2 and the magnetic excitation J2 [AT] (magnetic potential) of the magnetic quadrupole element 5 are adjusted such that the reference trajectory is not affected. The whole lens system acts to correct the X-direction chromatic aberration to zero. Similarly, the potential φq3 [V] at the electrostatic quadrupole element 3 and the magnetic excitation J3 [AT] of the magnetic quadrupole element 6 are adjusted such that the reference trajectory is not varied. The Y-direction chromatic aberration is corrected to zero over the whole lens system.
Correction of the second-order aperture aberration using a hexapole element is next described. Under ideal conditions, the second-order aperture aberration should not be produced. However, because of mechanical accuracy limitations, the second-order aperture aberration is produced in practice in a parasitic manner on the aberration corrector C. First, the second stage of multipole element 2 is operated as a hexapole element. The second-order aperture aberration in the X-direction is corrected to 0 by the potential φS2 [V] at the hexapole element over the whole lens system. The electrostatic octupole element 13 is operated as a hexapole element. The second-order aperture aberration in the Y-direction is corrected to 0 by the potential φS3 [V] at this hexapole element. Then, the second-order aperture aberration in a direction that is a combination of the X- and Y-directions (e.g., in a direction shifted by 30° or 60° from the X-axis) is corrected to 0 by operating the first stage of quadrupole element 1 and the fourth stage of multipole element 4 as hexapole elements.
Correction of spherical aberration (correction of the third-order aperture aberration) is next described. Where spherical aberration is corrected, X- and Y-direction chromatic aberrations are corrected. Then, the X-direction spherical aberration in the whole lens system is corrected to zero by the potential φ02 [V] at the electrostatic octupole element 12. The Y-direction spherical aberration is corrected to zero by the potential φ03 [V] at the electrostatic octupole element 13. Then, the spherical aberration in the resultant direction of the X- and Y-directions is corrected to zero by the electrostatic octupole elements 11 and 14 in the first and fourth stages, respectively. In practice, repeated mutual adjustments are necessary.
Potential or voltage φ used in the following description regarding electrostatic multipole elements indicates a positive (+) value of the multipole elements arranged normally as shown in FIGS. 4A and 4B. Similarly, magnetic excitation J of the magnetic type indicates magnetic excitation [AT] on the positive (+) side.
A scanning electron microscope is shown in FIG. 5 as one example of a charged-particle beam system using the above-described aberration corrector C. In FIG. 5, an electron gun has an emitter 21 emitting an electron beam 22, which enters the aberration corrector C through plural lenses 23. At this time, the lenses 23 control the beam 22 hitting the corrector C to focus the beam transmitted through the corrector C onto the surface of a specimen 25 by an objective lens 24. This corrector C is the aberration corrector shown in FIG. 1.
The electron beam is scanned over the surface of the specimen 25. Secondary electrons ejected from the surface of the specimen are detected by a detector in synchronism with the scanning. A scanned secondary electron image of the surface of the specimen is displayed on the viewing screen of a cathode-ray tube (CRT) by supplying the output signal from the detector as a brightness modulating signal for the CRT in synchronism with the scanning of the beam over the specimen.
In the above-described scanning electron microscope, the third-order spherical aberration and first-order chromatic aberration are corrected by the fact that the electron beam transmitted through the aberration corrector C already described in connection with FIG. 1 passes through the given X- and Y-trajectories. In this way, the resolution of the scanning electron microscope can be improved greatly.
After the third-order spherical aberration and the first-order chromatic aberration have been corrected by the aberration corrector C described above, aberrations limiting the resolution of the scanning electron microscope are the fifth-order spherical aberration and third-order chromatic aberration. These aberrations are known as higher-order aberrations and cannot be corrected by the aforementioned aberration corrector.
Methods of correcting the higher-order aberrations are described in detail in U.S. Pat. No. 6,924,488 and JP2004-087460. In particular, plural transfer lenses having a function of projecting an aberration generation point present near the rear end of an aberration corrector onto the front focal point of the objective lens are disposed between the aberration corrector C and the objective lens 24.
An example of the structure for correcting higher-order aberrations described in the patent references is shown in FIG. 6, where there are shown plural transfer lenses, i.e., a first transfer lens 31 and a second transfer lens 32. A point 33 at which aberration is produced (hereinafter may be referred to as the aberration generation point) is projected onto the front focal point 34 of the objective lens 24 using the plural transfer lenses 31 and 32, thus correcting the higher-order aberrations. Also shown in FIG. 6 are an object point 35, the primary trajectory of an electron beam indicated by solid line 36, an aberration trajectory indicated by broken line 37, the principal plane 38 of the objective lens, the focal distanced of the transfer lenses, and the front focal distance fo of the objective lens 24.
It is also possible to correct the higher-order aberrations using only one stage of transfer lens instead of plural transfer lenses. A structure using this single stage of transfer lens is also described in detail in the above-cited patent references. This structure is shown in FIG. 7. Like components are indicated by like reference numerals in both FIGS. 6 and 7. Detailed description of already described components is omitted. Shown in FIG. 7 are the single stage of transfer lens 40 and a transfer point 41.
The principle of correcting the higher-order aberrations using the transfer lens is described in detail, for example, in Nucl. Instr. and Meth. In Phys. Res. A 519, 264-279 (2004). The principle can be understood from the forms of the following two composite aberration coefficients, i.e., fifth-order spherical aberration coefficient C5s and third-order chromatic aberration coefficient C3c:
                              C          ⁢                                          ⁢          5          ⁢          s                =                  3          ⁢                                    Cs              2                                      f              0              2                                ⁢          Lh                                    (        1        )                                          C          ⁢                                          ⁢          3          ⁢          c                =                  4          ⁢                                    Cs              ⁢                                                          ⁢              Cc                                      f              0              2                                ⁢          Lh                                    (        2        )            
In Eqs. (1) and (2) above, Cs and Cc are the third-order spherical aberration coefficient and first-order chromatic aberration coefficient, respectively, of the objective lens 24. Lh is the distance between a transfer point 41 at which an aberration generation point 33 is projected by the transfer lens 40 and the front focal point 34 of the objective lens 24. f0 is the front focal distance of the objective lens 24. As can be seen from Eqs. (1) and (2), when the distance Lh decreases down to 0, the coefficients C5s and C3c become null. This state can be realized by optimizing the strength and position of the transfer lens such that the transfer point 41 becomes the front focal point 34 of the objective lens 24 as shown in FIG. 7.
However, in order to realize the correction conditions under which Lh=0 is satisfied, using the plural transfer lenses 31 and 32 already described in connection with FIG. 6, restrictive conditions are imposed on the positions and strength of the transfer lenses 31 and 32. That is, as shown in FIG. 6, to permit a collimated electron beam emerging from the aberration corrector C to hit the objective lens 24 in a collimated manner via the two stages of transfer lenses 31 and 32 and to project an aberration generation point 33 at the front focal point 34 of the objective lens 24, the positional relationship between the two stages of transfer lenses 31 and 32 is restricted by the focal distanced of the transfer lenses as shown in FIG. 6.
Accordingly, there is the problem that use of the two stages of transfer lenses increases the length of the microscope column of the scanning electron microscope. That is, a length of about 4 ft is required. For example, where it is assumed that ft=200 mm, a length of 800 mm is necessary. This is undesirable where external disturbances, such as vibrations, are taken into account. Usually, a scanning lens for scanning the electron beam and a secondary electron detector for detecting secondary electrons produced from the specimen are disposed between the transfer lens 32 and the objective lens 24. Therefore, it is difficult to reduce the length. Where the transfer lens 40 is made of a single stage as shown in FIG. 7, the primary trajectory 36 of the beam is varied by the position and strength of the transfer lens 40. Consequently, there is the problem that the optimum conditions for the whole system are complicated.